IT WAS NOT A CRUNCH OR A MOAN BUT a horrified hush spreading through the crowd that signaled the ghastly instant. On the Astroturf at Giants stadium, Jets defensive lineman Dennis Byrd lay motionless, unable to move his hands or legs. With all the power of his 266 lbs. of hurtling flesh, Byrd had unintentionally rammed his helmeted head into the chest of his 275-lb. teammate Scott Mersereau. The impact crumpled a vertebra in Byrd's neck, crushing part of the underlying spinal cord as well as plunging dagger-like slivers of bone into the soft, vital nerve tissue.